BEIJING - Some foreign companies in China exploit their workers by forcing them to do overtime or underpaying them, the labour minister said on Wednesday, as controversy swirls over working conditions at a plant that makes Apple Inc's iPhones and iPads.

Apple has faced a slew of bad press following deaths and reports of suicides at its China supply firms.

Three workers at Foxconn Technology Co Ltd died in a blast last year when dust from polishing iPads ignited, and labour rights groups have said 18 workers at Foxconn sites killed themselves, or tried to, in 2010.

The vast majority of Foxconn's 1.2 million employees are involved in assembling Apple products, according to media reports.

Labour Minister Yin Weimin said foreign companies generally followed the law when it came to their workers, but added that problems did exist.

"Foreign-invested companies have made great contributions to China's economic development and have played a positive role,"he told reporters on the sidelines of the annual meeting of parliament.

"Of course, we have also noticed that problems exist at some companies, for example excessive overtime, too low pay for some workers and a lack of concern for people," Yin said, without naming any companies.

"Relevant government departments and unions will increase their supervision and management when it comes to these issues."

Firms found to be breaking the law would be "severely investigated and dealt with".

Rights activists say many abuses also occur at Chinese companies, and the problem is particularly hampered by a government ban on independent labour unions and restrictions on the right to organise and strike.

The government will "step up guidance on companies' use of workers, supervise and push for improvements in working and living conditions and step up their concern for people", Yin said, without elaborating.

The Washington D.C.-based Fair Labor Association is currently carrying out an Apple-sanctioned extensive study of work conditions at Apple's top eight suppliers in China, including Foxconn, the world's largest manufacturer with close to 1.2 million workers in China alone.

Apple CEO Tim Cook has described the probe as an"unprecedented" audit to mitigate longstanding criticism of the maltreatment of workers at some suppliers.

Working conditions at Foxconn, whose flagship unit is Taiwan's Hon Hai Precision Industries group, have in particular been a constant thorn in the company's side.